This invention relates to the purification of 1,3-bis-(4-chlorobenzylideneamino)guanidine hydrochloride (robenidine hydrochloride). More particularly, this invention relates to the purification of 1,3-bis(4-chlorobenzylideneamino)guanidine hydrochloride by selectively converting a contaminant salt to the corresponding free base which is soluble in the reaction medium. Still more particularly, this invention relates to the purification of 1,3-bis(4-chlorobenzylideneamino)guanidine hydrochloride by selectively converting any 1,2,3,tris(4-chlorobenzylideneamino)guanidine hydrochloride therein to the free base and recovering the 1,3-bis(4-chlorobenzylideneamino)guanidine hydrochloride in a higher state of purity.
Robenidine, usually administered as the hydrochloride, is a coccidiostat for the treatment of chickens. U.S. Pat. No. 3,769,432 to Tomcufcik, the full disclosure of which is hereby incorporated by reference, discloses the preparation and use of robenidine hydrochloride. The preparation is carried out by the reaction of N,N'-diaminoguanidine hydrochloride (I) and 4-chlorobenzaldehyde (II) as shown below: ##SPC1##
This process for the production of robenidine hydrochloride (III) results in a product which is pharmacologically acceptable. However, depending on the amount of N,N',N"-triaminoguanidine hydrochloride impurity present in the starting N,N'-diaminoguanidine hydrochloride, the product may have to be repeatedly recrystallized in order to lower the concentration of 1,2,3-tris(4-chlorobenzylideneamino)guanidine hydrochloride (IV) ##SPC2##
contaminant to an acceptable level. This process of Tomcufcik usually results in a product having a melting point of 289.degree.-290.degree.C., with decompostion, and contains from about 1% to 3% by weight of 1,2,3-tris(4-chlorobenzylideneamino)guanidine hydrochloride after recrystallization from aqueous alcoholic solution. However, the product can contain as much as 7% of 1,2,3-tris(4-chlorobenzylideneamino)guanidine hydrochloride if the starting material (I) is very impure.